1. Field of the Invention
The present invention relates generally to amplifier systems that operate in forward and reverse modes.
2. Description of the Related Art
Gain programmable power amplifiers are essential in a variety of modern communication systems. In cable modem systems, for example, communication signals are transmitted xe2x80x9cdownstreamxe2x80x9d from a cable headend for reception by system subscribers and data signals are transmitted xe2x80x9cupstreamxe2x80x9d from the subscribers. The downstream signals are generally transmitted in one frequency band (e.g., 45-800 MHz) and the upstream signals in a different frequency band (e.g., 5-43 MHz).
When transmitting data signals upstream in a xe2x80x9cforwardxe2x80x9d transmit operational mode, an upstream amplifier preferably operates with minimal signal distortion, suppresses generation of transient signals, and provides an output impedance Zout that closely matches a cable impedance (e.g., 75 ohms). In order to avoid avoid degradation of a corresponding downstream amplifier, the upstream amplifier preferably continues to provide the output impedance Zout in a xe2x80x9creversexe2x80x9d power-down operational mode.
Conventional power amplifiers have typically attempted to meet these demanding requirements with amplifier structures (e.g., transformers and field-effect transistor (FET) switches) that impose significant cost and volume penalties and/or are not compatible with other integrated-circuit fabrication processes.
The present invention is directed to amplifier systems and methods which closely approximate a constant output impedance and a constant quiescent output signal during forward and reverse modes of operation. These goals are realized with small, inexpensive components that are compatible with integrated-circuit fabrication processes.
In a system embodiment, a signal amplifier transitions from an enable state to a disable state in response to a signal-amplifier disable signal DSsa, a feedback path is coupled to the signal amplifier to reduce its impedance to a substantially lower enable state output impedance and a reverse amplifier transitions to an on state in response to a reverse-amplifier enable signal ESra and is coupled to the feedback path.
The reverse amplifier thus interrupts the feedback path to thereby replace the enable state output impedance with the signal amplifier""s higher impedance and forms, with the feedback portion, a disable state output impedance that approximates the enable state output impedance. A signal generator provides the signal-amplifier disable signal Dsa and the reverse-amplifier enable signal ESra with the reverse-amplifier enable signal ESra initiated before the signal-amplifier disable signal DSsa and terminated after the signal-amplifier disable signal DSsa.
Another system embodiment includes an error-current canceler that comprises a current sensor which generates a sense signal in response to an error current Ierr that is generated by the signal amplifier as it transitions between its enable and disable states and further comprises a correction-current generator that responds to the sense signal by canceling the error current Ierr with a substantially equal and opposite correction current Icrctn.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.